1. Field of the Invention
The present invention relates generally to the field of electric motors and stator windings for such motors. More particularly, the invention relates to a novel technique for forming coils for electric motor stators and for installing and interconnecting a multiplicity of such coils in an electric motor.
2. Description of the Related Art
Electric motors of various types are omnipresent in industrial, commercial and consumer settings. In industry, such motors are employed to power all types of rotating machinery, such as pumps, conveyors, compressors, fans and so forth, to mention only a few. Conventional alternating current electric motors may be constructed for single or multiple phase operation, and are typically specifically designed to operate at predetermined synchronous speeds, such as 3600 rpm, 1800 rpm, 1200 rpm and so on. Such motors generally include a stator, comprising a multiplicity of coils, surrounding a rotor which is supported by bearings for rotation in the motor frame. Alternating current power applied to the motor causes the rotor to rotate within the stator at a speed which is a function of the frequency of alternating current input power and of the motor design (i.e., the number of poles defined by the motor windings and rotor resistance). The rotor shaft extends through the motor housing and is connected to elements of the machinery driven by the electric motor.
In conventional alternating current electric motors, stator winding coils are disposed in parallel slots formed around the inner periphery of a stator core. Certain of the winding coils are electrically connected in groups around the stator core to establish the desired electromagnetic fields used to induce rotation of the rotor. The number and locations of the windings in the stator core generally depends upon the design of the motor (e.g., the number of poles, the number of stator slots, the number of winding groups, and so forth). For example, one common stator design for a four-pole, three-phase motor includes 48 stator slots in which 12 groups of windings are installed, each group consisting of 4 coils.
Each stator winding coil is typically preformed in a diamond, elongated hexagonal or in an oval shape and pressed into the appropriate slots during assembly of the stator. Each winding coil includes a number of turns of wire which loop around end or head regions of the stator between the slots in which the winding coil is installed. Following installation in the slots, the coils in each group are generally pressed into a bundle at either end of the stator, and sheet insulating material is provided between power phase windings. Once complete, the entire assembly is packaged and varnished to bond and environmentally protect the winding. The varnish displaces the air between the coils and, in effect, reduces the risk of ionization.
While conventional motor stators and motors incorporating such stators function well in many applications, they are not without drawbacks. In particular, such stators may encounter problems in an increasing number of industrial applications wherein motor drive circuitry includes electronic inverter circuits, such as in variable frequency AC drives. Such inverter circuits are useful for varying the frequency of the electrical power used to drive the motors, and hence, their driven speed.
As inverter drives have been improved over the years, extremely rapid inverter switching has been attained which aids in providing precisely controlled wave forms, but which can result in periods of very high potential difference between stator winding coils. The magnitude of these potential differences depends upon the stator configuration. For example, for the four-pole, three-phase, 48 slot stator, having 12 groups of 4 winding coils, mentioned above, when the windings are coupled in a wye configuration, each coil experiences a voltage drop of only 1.717 times the line voltage, divided by 16. If however, the winding groups are connected in a delta configuration, each coil experiences twice that voltage drop. If the 4 coils in each group are coupled in parallel in the delta configuration, as may sometimes be required, each group will experience a total voltage drop equal to the line voltage. When coupled to an inverter drive, the potential difference between grouped coils in the stator may, during transient operating periods, rise as high as three times the inverter drive bus voltage due, in part, to a reflected wave phenomena resulting from the very high switching rates of the inverter circuit components and inverter drive-to-motor lengths. Thus, for inverter drives having a direct current bus voltage of approximately 650 volts, transient potential differences experienced between certain coils in the groups may rise as high as 1,950 volts. Under such conditions, conventional stator windings having grouped coils pressed into a bundle at the stator ends may be severely degraded by corona ionization resulting from the intense field gradients induced by these high voltages. Such ionization may lead to breakdown of the insulation wire enamel between the coil windings ultimately reducing the life of the motor.
There is a need, therefore, for an improved technique for forming electric motor stators which is effective in limiting the adverse effects of high potential differences between the stator windings, particularly for inverter drive applications. Moreover, there is a need for a technique which is capable of physically and electrically isolating stator windings from one another sufficiently to avoid corona ionization and the consequent degradation of the windings and insulation that ultimately results.